Avidin is a protein found in egg whites and contains four subunits. Biotin is a stable, water-soluble vitamin. Biotin and avidin interact specifically under mild and certain harsh conditions to form a strong, stable, avidin-biotin complex in which each of the four subunits of avidin bind a biotin molecule. This binding persists when biotin is attached by means of its carboxyl group to another molecule, or when avidin is attached to another molecule. For example, biotin may be secured or attached to molecules on the surface of a cell or to anticellular antibodies which have been reacted onto a cell, and then subsequently is reacted with a ferritin-avidin conjugate, to provide a method for localization studies in affinity cytochemistry (see, for example, Trends in Biochemical Science, 3, N257 (1978), hereby incorporated by reference). Biotinyl-antibody and conjugated avidin products (with fluorescein, rhodamine, ferritin or horse radish peroxidase) are offered commercially, to provide investigators with reagents for studying biochemical and immunochemical structures or processes; for example, the location or extent of cell-surface substances.
A modified avidin-biotin system has been employed to enhance immune cellular agglutination of erythrocytes (see Clinical Chemistry, 25, No. 9, 1572 (1979), hereby incorporated by reference). Biotin or caproylamidobiotin was either attached directly to the cells or indirectly using biotin or caproylamidobiotin-anticellular antibody. The addition of avidin then achieved agglutination, and a biotin or caproylamidobiotin-conjugated macromolecule was added as an extender in conjunction with more avidin, to enhance the agglutination.